softpipe: Grab a ref when the fb is set.

softpipe: Grab a ref when the fb is set.

Nasty bug when the surface is freed and another is allocated right on
top of it. The next time we set the fb state SP thinks it's the same
surface and doesn't flush, and when the flush eventually happens the
surface belongs to a completely different texture.

(cherry picked from commit a77226071f6814a53358a5d6caff685889d0e4ec)

Conflicts:
	src/gallium/drivers/softpipe/sp_context.c